An example of a system for obtaining traffic information is a system in which the presence of a vehicle is detected. Such detection systems are already known.
A usual detection system comprises a light source and a light detector for respectively transmitting and receiving a light signal. The light source is here usually placed above the vehicle and the detector is situated on the road surface. Detection of a vehicle takes place in that the vehicle interrupts the optical connection between the light detector and the light source.
Because the light source is situated some distance from the detector, these sources and detectors are generally provided with a lens system for focussing the light. If this were not the case, it would be necessary to use high-power light sources in order to still have sufficient signal strength at the position of the light detector. Such sources have a limited lifespan, have rigorous requirements in respect of heat management and consume a great deal of energy.
A drawback of the focussing of light is that it is highly sensitive to dust and/or other small particles which accumulate on the lens system. There is therefore the danger of a vehicle being wrongly detected. It is likewise necessary in such systems to align the light source and light detector relative to each other.